When transmitting speech signals/sound signals through a packet communications system, as exemplified by Internet communications, or through a mobile communications system, and/or the like, compression techniques/coding techniques are often employed to improve transmission efficiency for speech signals/sound signals. While on the one hand, speech signals/sound signals are simply coded at low bit rates, there are growing needs for techniques that code speech signals/sound signals of wider bands, as well as techniques that carry out coding/decoding with small computation amounts without degrading sound quality.
In response to such needs, various techniques that reduce computation amounts without degrading the quality of the decoded signal are being developed. By way of example, with the technique disclosed in Patent Literature 1, computation amounts for pitch period searches (adaptive codebook searches) are reduced in connection with a Code-Excited Linear Prediction (CELP)-type coding apparatus. Specifically, the coding apparatus sparsifies the updating of an adaptive codebook. With respect to the processing method for sparsification, a method is adopted where the value of a sample is replaced with zero (0) when the amplitude of the sample does not exceed a given threshold. Thus, computation amounts are reduced by omitting, at the time of a pitch period search, processing (specifically a multiplication process) for parts where the value of the sample is 0. In addition, there is disclosed a feature where the above-mentioned threshold is made adaptively variable from process to process, as well as a feature where samples are sorted in descending order of absolute value, and where the sample value is replaced with zero (0) for all samples that fall outside of a predetermined number of samples from the top.